Thermal hydraulics and fuel integrity in spent fuel
dry cask interim storage facility (SPEFU)
Risto Huhtanen
•
Asko Arkoma
VTT Technical Research Centre of Finland, Espoo
Introduction / background
Fuel behaviour during dry storage is actively investigated,
especially now that in many countries interim storage times are
foreseen to be extended to 100 years. The fuel behaviour models
include irradiation integrally, and as such are not necessarily
applicable to the interim storage conditions. The possible
damage mechanisms include creep rupture, as well as hydrideinduced effects. The hydrides also reduce the ductility of the fuel
Thermal analysis of single assembly
A proper simplification method is sought for modelling the heat
transfer of the fuel rod bundle. In the suggested method reduction
of computational cells is about 96%. The detailed rod bundle is
modelled with solid block where the local heat conduction
depends on local temperature. The modified heat conductivity is
calculated from the geometrical parameters of the rod bundle.
The accurate simulation has got 1.38 million cells in the
computational grid. The amount of cells in the coarse grid is
only 4% of that. The temperature field in one cross section is
shown in Figure below. Diagonal temperature profile is shown in
experimental case and solid block simulation. Total height of the
rod assembly is about 4 m.
cladding, possibly creating challenges during the fuel handling
phase.
This reasearch includes material part for evaluating the cladding
integrity under dry storage conditions and thermal analysis for
evaluating the prevailing conditions during the storage period.
Modelling of creep in dry storage
A fuel behaviour code should be able to model the behaviour of a
fuel rod during all the stages of operation: irradiation, water pool
cooling (~5 years), one day of drying, and finally the dry storage.
In order to be able to model fuel rod under dry storage, the creep
model needs to be updated. Two dry storage creep models were
found from open literature, developed by EDF/CEA and CIEMAT,
and those were implemented into a single rod fuel performance
code ENIGMA used at VTT.
At the moment, the results produced by the two creep models
show too high creep values during the dry storage to be realistic,
and that will require further analysis of the creep laws and their
implementation. Compared with the usually applied limit of 1%
maximum hoop strain during the dry storage, the obtained
calculation results are higher as seen in figure below.
Cask with cooling fins
The proposed method of using modified thermal conductivity for
the air in the cooling fin domain takes into account the effect of
the conducting cooling fins without the need to include all
geometrical details into the model. In this example the number of
computational cells was 8% of the amount in the fine grid case.
This will be reflected to the computational work needed to solve
the problem. The simplified method can be used for solving the
problem without compromising the accuracy too much.
Fine grid
Cladding creep hoop strain calculated with ENIGMA using the steady-state
power history found in an article by CIEMAT (L.E. Herranz, F. Feria, 2010.
Progress in Nuclear Energy, Vol. 52, Issue 7, pp. 634-639). The increase in
creep due to dry storage is 0.5% according to Herranz and Feria,
calculated with FRAPCON code, while with VTT-ENIGMA, the increase is
1.3% when applying the CIEMAT creep model .
Coarse grid
The effect of hydrides
°C
m/s
A literature study was made in order to gather and summarize
information on the effects of hydrides on cladding integrity in
dry storage. Results from both experimental research and
computational analyses are elaborated, and the current stateof-the-art of research in this field has been brought out.
Hydride re-orientation and loss of ductility due to hydrides are
among the most topical research subjects.
Contacts
Risto Huhtanen
[email protected]
Temperature surface 30°C
Asko Arkoma
[email protected]